You can try the following. Create a custom terminal that represents any erroneous string. In its TryMatch method, consume all until next whitespace. Set its priority to (LowestPriority) so it is called to produce token only when all other terminals fail
to produce token. Then in query conversion recognize this terminal and handle it in proper way (ignore or assume it TRUE in "AND" expressions). You may need to play with priority, as ImpliedSymbol terminal also has lowest priority and they may interfere.